Mechanical Performance Optimization of Bonded Composite Patch Repairs: a flexural analysis

• Autor(a) principal: Ferreira, Francisca Sá
• Data de Publicação: 2020
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10400.6/11682
• topic: Compósitos; Fadiga; Fibras de Carbono; Flexão; Impacto; Laminado de Controlo; Reparação Em Remendo; Sensibilidade Ao Furo; Sequência de Empilhamento; Domínio/Área Científica::Engenharia e Tecnologia::Engenharia Aeronáutica

The effect of the stacking sequence ([0,90,0,90,0,90]S or [90,0,90,0,90,0]S) and patch size (30, 40 or 60 mm) were studied, as well as the notch sensitivity of the parent plate to a 20 mm diameter hole. For this purpose, quasi-static three-point bending tests were performed to evaluate the bending stiffness and load-carrying capacity of the repaired structure. The FE numerical analysis was validated. The specimens were also low-velocity impacted in order to evaluate the influence of the studied variables in the damage mechanisms and residual properties. The specimens tested in three-point bending fatigue to verify the tendency of the fatigue effects depending on the patch dimensions. It was observed that the parent laminate, for both stacking sequences, is notch sensitive according to the studied conditions. For the specimens with 0º outermost ply, the 60 mm patch was revealed as an effective repair as its mechanical properties were restored. However, none of the studied patch sizes were considered as a valid repair for the 90º specimens. The impact damaged relative area, caused by an energy level of 12J, was inversely proportional to the increase of patch size,for both orientations, as the 60 mm patch presented a damage under 25% of the overall patch area, while the 30 and 40 mm presented damage in 38 to 50% of the overall patch area. The fatigue degradation, when exposed to fatigue loading, was found to be proportional to the patch size, as the 30 mm patches for both laminate types presented longer life cycles, 3933 for the 0º specimens and 6599 for the 90º cycles, while for the other patch sizes the maximum of life cycles were around 1667. The overall damage mechanisms were found to be consistent for the quasi-static and dynamic tests, as for the 0º specimens the ultimate failure was found to be the interlaminar delamination in the patch and, for the 90º specimens, the delamination was verified in the outermost ply of the parent laminate, due to the patch and adhesive pulling the ply yarns oriented at the transverse direction. Finally, the adhesive was proven not to be the weakest link in any of the repairs, as the failure damage was always verified in one of the composite laminates.